Process for the production of powdered metals

ABSTRACT

AN APPARATUS AND PROCESS ARE PROVIDED FOR REDUCING METAL HALIDES TO METAL POWDER LOW IN HALIDE CONTENT, THE PROCESS COMPRISING PROVIDING A PREHEATED SUBSTANTIALLY UNIFORM MIXTURE OF REACTANTS COMPRISING A METAL HALIDE UNIFORMLY DISPERSED THROUGH HYDROGEN AT A TEMPERATURE ABOVE THE REACTION TEMPERATURE FOR SAID REACTANTS, AND THEN IMMEDIATELY FEEDING THE PREHEATED MIXTURE INTO A REACTION CHAMBER, WHEREBY THE REACTION TO METAL POWDER IS SUBSTANTIALLY SPONTANEOUSLY EFFECTED.

June 20, 1972 K. A. JONSSON 3,671,220

PROCESS FOR THE PRODUCTION OF POWDERED METALS Filed May 11, 1970 HEATING7 g u/wr comm T INVENTOR.

F l G. 2 if/4159f? Jan 55W United States Patent O US. Cl. 75-.5 B 9Claims ABSTRACT OF THE DISCLOSURE An apparatus and process are providedfor reducing metal halides to metal powder low in halide content, theprocess comprising providing a preheated substantially uniform mixtureof reactants comprising a metal halide uniformly dispersed throughhydrogen at a temperature above the reaction temperature for saidreactants,- and then immediately feeding the preheated mixture into areaction chamber, whereby the reaction to metal powder is substantiallyspontaneously effected.

This invention relates to a process and apparatus for the production ofmetal powders by the reduction of metal halides and, in particular, tothe production of such metal powders as tungsten, molybdenum, tantalum,nio bium, rhenium, chromium and alloys thereof. A

PRIOR ART AND THE PROBLEM CONFRONTI-NG THE ART It is known to producetungsten powder by the hydrogen reduction of W by employing a batchoperatlon in which powder W0 is placed in smallboats following which theboats areheated to the desired reducing temperature in hydrogen. Thisprocess has its disadvantages in that the labor requirements arehighaand that large amounts of hydrogen gas are generally necessary. I

As illustrative of one process disclosed in the literature, gaseouschlorides of heavy metals are reduced with a reducing agent, such ashydrogen gas. .In this process, the

two gases are fed through concentric tubes to a reaction chamber in suchaem anner that the .gases are mixed together only when they reach thereaction furnace proper beyond the exit orificesor' mouths of the tubes.However, this process is disadvantageous in that metal reduced out tendsto form deposits at the tube orifices on the wallcommon to theconcentric tubes. In an attempt to prevent the formation of suchdeposits, an additional concentric tube for supplying inert gas to thereaction chamber was proposed inserted between the tube for the metalchloride and the hydrogen tube. However, this expedient did not providesatisfactory mixing of the reaction components, the mixing being evenfurther adversely affected, and, moreover, incomplete reduction resultedwherein the powder produced had a high chloride content. In addition,the metal tended to partially reduce out in the form of metal foil onthereactor walls which appeared as broken pieces (glittering fragments)in the powder produced which adversely affect the quality of the powder.

It was difiicult with the foregoing process to measure the temperatureof the gas mixture resulting from the flow through the concentric tubesprior to the entry of the mixture into the reactor. Generally speaking,the measurement was usually limited to measuring the temperature of thegas components before entry into the chamber. Because of this, it isvery difiicult tomaintain an accurate control of the temperature of themixture, which temperaturecontrol is an important parameter forobtaining and controlling the desired grain or particle size.

ice

Another known process comprises applying metal coatings to objects suchas tungsten or molybdenum coatings. In this process, the gaseousreactants are mixed in a single tube with the conditions of reductionchosen to occur at the surface of the object to be coated. For example,the gas mixture is maintained at below the reaction temperature whilethe object is heated, e.g. inductively heated, to a temperature abovethe reaction temperature of the mixture to effect reduction at thesurface of the object. As is quite apparent, the object here is not toform metal powder but to reduce out a metallic coat on the surface ofobjects.

In US. Pat. No. 3,450,525, a method is disclosed for producing metalpowders in which metal halides are gasified in a pre-heating furnace andthen mixed with hydrogen at a temperature below the reaction temperatureof the mixture, following which the gas mixture is directed into areaction chamber where heat is supplied at the walls thereof atsubstantially above the reaction temperature to avoid the formation ofmetal foil deposits on the walls. Because the reaction temperature isreached first within the reaction chamber by heat supplied by the walls,the process first proceeds relatively slowly and incompletely, whichresults in a relatively high content of residual halides in the metalpowder produced and in the requirement of relatively large excess ofhydrogen. An essential disadvantage of this process is that it isdifiicult to control the grain size of the resulting metal powder.

The present process differs in that metal powder of the desired grainsize low in residual halide content can be produced by using lesshydrogen than is normally employed in prior art processes.

OBJECTS OF THE INVENTION It is thus the object of the invention toprovide a process for the production of metal powder from metal halides,including oxyhalides.

Another object of the invention is to provide a process for producingmetal powder from metal halides, such as chlorides of tungsten,molybdenum, tantalum, niobium, rhenium, chromium, and the like, byreduction with hydrogen with the following advantages: (1) control ofgrain size of the resulting metal powder; (2) use of lower amounts ofhydrogen; (3) inhibition of metal foil formation on reactor walls, infeed lines and at the nozzles; and (4) production of metal powder with alow residual halide content.

These and other objects will more clearly appear from the followingdisclosure and the appended drawing, wherein:

. FIG. 1 is a schematic representation of one apparatus embodiment-forcarrying out the invention; and

FIG. 2 is afragment in cross section of a mixing tube.

SUMMARY OF THE INVENTION Stating it broadly, a process is provided forreducing metal halides to metal powder low in residual halide contentcomprising, providing a preheated substantially unreacted mixture ofreactants comprising a metal halide uniformly dispersed through hydrogenpreheated to a temperature at least above the reaction temperature forsaid mixed reactants, and then immediately feeding the preheated mixtureinto the reaction chamber, whereby the reaction to metal powder issubstantially spontaneously effected.

For example, in the case of the chlorides of tungsten, molybdenum,tantalum, rhenium, chromium, and the like metals, the reduction processis carried out in the 'presence of hydrogen in such a manner that thechlorides, in preferably the gaseous state, are mixed with hydrogen inone or several ducts at a first temperature above the reactiontemperature before the mixture is fed into the reactor, the preheatedmixture being thereafter directed at a predetermined or controlledvelocity into the reactor which is preferably, though not necessarily,heated to a second temperature above the first temperature The resultingmetal powder is then separated from the gas. The desired grain size ofthe powder product is determined by controlling one or more variables ofthe gas mixture, such as the temperature, the velocity of the gaseousreactants, the ratio of the reactants, etc, The apparatus employed forcarrying out the process preferably utiliz'es a metal mixing tube whichextends into the reactor, the apparatus being provided with means foradjusting the ratio of the mixture, the velocity of the mixture, the

DESCRIPTION OF THE DRAWING The schematic depcited in FIG. 1 shows areactor 1 provided with a plurality of electric heating elements 2, theheating elements being divided in groups (known in the art) so that thethermal energy is supplied in a series of zones. A metal mixing duct 3is provided which opens into the reactor, the mixing tube extendingsufficiently inwardly from reactor end wall 1A in order to avoidreaction between the halide gas and the reduction gas at the end wall.

The metal duct preferably is mounted coaxially 'With the longitudinalaxis of the reactor. However, the reactor may also be provided withseveral mixing ducts for the supply of mixed gas. In such cases, theducts are preferably introduced into the reactor near or about the lineof symmetry or longitudinal axis of the reactor and in parallel withsaid axis. V

The reaction components in the gaseous state and possibly another gas(e.g. an inert gas) are supplied to the mixing tube through feed lines.The drawing shows a line 4 for the supply of metal halides in preferablythe gaseous phase, a second line 5 being provided for sup.- plyingreduction gas (e.g. hydrogen), and a third line 6 for supplying, whendesired, an inert gas, or a halogen in gaseous state or some other gas.A plurality of heating are t a e m ned. at ,.tem e tur d velocity suchthat a powder of a desired grain size is obtained. It is, therefore,important that the gas velocity should be selected relative to thelength of the mixing tube so that there is sufficient time for only aslight reaction to begin within the mixing tubeiprior'tothe entry of thegas into the reactor. In a sense,-the reaction incubatesinthe mixingtube just before it enters there'action zone in the reaction charnber,Subsequent to the-entry thereof into the reactor, the gasm'ixturesatisfies all conditions required for a continued reaction, which takesplace very rapidly and completely in the reactor which preferably,though not necessarily, is maintained at a higher temperature than thetemperature in the mixing tube.

The metal powder obtained according to conventional processes in whichsubstantial excess of hydrogen is employed usually is much toofine-grained, and, in most cases, falls within the grain size range fromabout 0.01 to 0.05 micron with the density between about 0.15 and 0.35g./cm. The range whichvis of interest in commerce, however,, liesbetween 0.05 and 10 microns, and the densities of interest are froni 0.8to 4 g./cm. In the present invention, by controlling the variables,powder may be obtained which falls within the aforementioned desiredranges. The ratio of mixture of reduction gas and halide gas affects thegrain size inthat an increase in the reduction gas content results indecreasing grain size. In

' accordance with the invention, asmall grain size can be elements areshown at 7 for possibly supplying heat to the gas. These elements arearranged around the mixing tube in heat transfer relationship therewithin the conventional manner. Means 8 are provided for cooling the gasmixture to the desired first temperature where this, is found necessary.At 9, is shown a thermocouple for measuring the temperature of the gasmixture before it enters the reactor. The thermocouple is mountedimmediately before the place of entry as it has been found to be asuitable point for temperature measurement. Vibratory means 10 areprovided for vibrating the mixing tube, the means being preferablyattached immediately before the point of entry of the mixing tube intothe reactor By subjecting the mixing tube to vibration, it is possibleto inhibit metal reduced out from depositing on the orifiec of themixing tube in the reactor. The shape of the orifice of the mixing tubein the reactor is shown in'FIG. 2, where the annulus of the tube ispointed or tapered so as to provide a sharp edge, to further avoidadhesion of the metal reduced out.

DETAIL ASPECTS OF THE INVENTION According to one embodiment of theinvention, the halide. gas and the reduction gas are mixed in one. orseveral tubes prior to being fed into the reactor at a temperatureexceeding the reaction temperature of the gas mixture. It is to beunderstood that the conditions for a reaction already prevail within themixing tube, and that the reaction is beginning to commerce here.However, it is not desirable that the reaction proceed to any greatextent within the mixing tube. The primary objective is to effect ahighly homogeneous mixture and, secondly, to produce a mixture in whichthe reactants achieved with a relatively small excess of reduction gaswhich, in the case of hydrogen, usually does not exceed twice thestoichiometric amount required for the reaction. This is advantageous inthat provides an economic process.

The temperature affects the grain size such that with increasingtemperature of the mixing gas, a. decrease in grain size is obtained.The grain size in the powder may be decreased more substantially byadding a halogen (e.g. chlorine) ,in the gaseous state to the gasmixture. This is because an indirect temperature increase results by thereaction of the halogen gas with excess hydrogen.

Inversely, larger grains in the powder are obtained by decreasing thetemperature of the gas mixture. Normally, thehydrogen gas and the metalhalide (e.g. chloride). gases are heated or cooled in advance to atemperature within the desired temperature range. Around the mixing tubemay then be supplied extra heating or cooling means for slightlyadjusting the temperature of the mixing gas to the desired range priorto the entry thereof into the reactor. Another method ofrapidlyadjusting the temperature of the mixing gas is by-dilution withaninert gas, such as nitrogen gas or hydrogen chloride. Such a dilution ofthe mixing gas results in a powder of a greater-grain size. I

The reaction-in the reactor, as already-mentioned, proceeds efiicientlyto completion, whereby the produced powder has a very low chlorinecontent, in many cases below 0.5%. A further advantage is the highlyimproved quality of the powder, due to the absence of inhomogeneity 'intheform of flakes or glittering foils. It is generallydesirable to'raisethe temperature in thelower reactor part by an additional supply of heatin order to assure completeness of the reaction.

According to the invention, it is also possible to coreduce chlorides,for example, tungsten chloride and rhenium chloride, and thereby producea metal powder in which substantially every grain is homogeneous from acompositional viewpoint.

The apparatus for carrying out the process comprises substantially amixing tube extending into a reaction chamber. There maybe a pluralityof mixing tubes,'but, irrespective of their number, they preferably areoriented 1 along the line of symmetry'or longitudinal axis of the more,preferably open within the reactor some distance inside from its innerwall. The mixing tubes preferably are made of a metal. If thetemperature of thegas mixture can be maintained below 600 C., tubes ofnickel or Inconel Cr. 6 to 7% 'Fe and'theb'alance nickel) may be usedwithout giving rise to aggravated corrosion problems which usually.contaminated the powder to an appreciable degree. Previously, tubes ofglass or quartz have been used, but such tubes had a very short lifeand, moreover, they contaminated the metal'powder when they boke intopieces.

Tubes made of metal are advantageous also in other respects. They can besubjected to vibrations, so that possible incrustations .on thetube caneasily be removed by shaking. This'can be effected very simply byplacing a vibrator on the mixing tube immediately before its entry intothe reactor. As stated hereinbefore, in order to prevent metal reducedout from adhering on the tube mouth in the reactor, the tube mouthpreferably is pointed so as to form a shap edge. It is also easy tosupply or remove heat through the metal tube, which facilitatescontrolling the temperature of the gas mixture.

As illustrative of the invention, the following examples are given:

Example 1 Utilizing an apparatus of the type shown in FIG. 1, a fiow of22 kg. WCl per hour was mixed with a flow of 96 litres of H per minute(referred to room temperature) in a nickel tube opening into the reactorheated to 1000 C. The WCl gas had a temperature of 400 C., and the H gasof 525 C. This resulted in a mixture which immediately prior to itsentry into the furnace had a temperature of about 440 C., since acertain amount of heat transfer between the mixing tube and the ambientenvironment had thereby taken place. The gas velocity was 25 m./sec. Thetungsten powder produced had a residual chlorine content of about 0.26%by weight and a grain size of about 0.2 micron determined by means of anelectron microscope. The density of the powder was 1.32 g./cm. Thepowder did not contain glittering metal fragments and was uniform, theinside of the mixing tube being free of metal coatings.

Example 2 A flow of 22 kg. WC1 per hour was mixed as in Example 1 with afiow of 70 litres of H per minute (referred to room temperature) in anickel tube into the reactor. The upper portion of the reactor washeated to 800 C., the central portion to 900 C., and the lower portionto 1000 C. The WCl gas had a temperature of 400 C., and the H gas atemperature of 150 C. This resulted in a mixture which immediately priorto its entry into the furnace had a temperature of about 320 C. Acertain amount of heat transfer between the mixing tube and the ambientenvironment had already taken place. The gas velocity was about 16 m./sec. The tungsten powder produced had a residual chlorine content ofabout 0.8% by weight and a grain size of 2.1 microns determined by meansan electron microscope. The density was 3.2 g./cm. The powder did notcontain glittering fragments and was uniform. The inside of the mixingtube was free of metal coatings.

Example 3 Utilizing an apparatus of the type shown in FIG. 1, a flow of13 kg. WC1 per hour and 3.0 kg. C1 per hour was mixed with a flow ofnitrogen gas of 96 litres/min. (referred to room temperature) in anickel tube opening into the reactor heated to 1110 C. The mixture ofWCl and C1 had a temperature of about 400 C. Subsequent to the admixingof the hydrogen gas, the temperature of the gas mixture was 415 C.immediately prior to the entry into the furnace. The gas velocity was 60m./sec. The tungsten powder produced had a residual chlorine content ofabout 0.5% and a grain size of about 0.05 micron determined by means ofan electron microscope. The powder did not contain glittering fragmentsand was uniform. The inside of the mixing tube was free of metalcoatings.

While the examples set forth hereinabove are directed to the reaction oftungsten hexachloride with hydrogen, the

reaction of other halides may be carried out similar to Examples 1, 2and 3.

Examples of the halide reactions which may be carried out similar toExamples 1, 2 and 3 are:

The amount of hydrogen which may be employed in carrying out thereaction need not exceed two times the stoichiometric amount. As statedhereinbefore, the grain size can be controlled according to the amountof hydrogen used, that is, the greater the amount of hydrogen, thesmaller the grain size. Similarly, the higher the mixing temperature ofthe unreacted ingredients, the smaller the ultimate grain size. Theaddition of a halogen to the gas mixture also results in a decrease ingrain size. Larger grains may be produced by inversely controlling oneor more of the foregoing variables, that is, by lowering thetemperature, or lowering the ratio of hydrogen to the halide, etc., orby adding an inert gas or HCl.

Although the temperature of the reactor in each of the examples ishigher than the mixing temperature of the reactants, the reactortemperature need not be higher so long as the reaction goes tocompletion therein and so long as the temperature of the substantiallyunreacted premixed ingredients fed to the reactor is above the reactiontemperature. Generally speaking, the temperature in the reactor isusually higher than the temperature of the unreacted gas mixture fed toit. The temperature in the reactor may range up to about 1600 C. and,for example, from about 700 C. or 800 C. to 1600 C.

The minimum reaction temperature of a particular mixture of metal halideand hydrogen can be determined by tests. For example, tests have shownthat heating WCl in a quartz tube furnace under a low hydrogen flowindicated that the minimum reaction temperature in this system rangedfrom about 300C .to 330 C.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What is claimed is:

1. A process for reducing metal halides to metal powder low in halidecontent which comprises,

providing a preheated substantially unreacted mixture of reactantscomprising a metal halide uniformly dispersed through hydrogen incubatedat a temperature above the reaction temperature for said reactants,

and then immediately feeding said preheated mixture into a reactionchamber,

whereby said reaction to metal powder is substantially spontaneouslyeffected in said reaction chamber.

2. The process of claim 1, wherein the metal halides are selected fromthe group consisting of halides of W, Mo, Ta, Nb, Re, Cr and mixturesthereof.

3. The process of claim 2, wherein the metal halide is tungstenhexachloride.

4. The process of claim 3, wherein the temperature of the preheatedtungsten hexachloride prior to feeding into the reaction chamber is overabout 300-330 C.

5. The process of claim 1, wherein the gr'ain' size of the ultimatemetal powder is determined by controlling at least one of theparameters: ratio of the reactants, velocity of the reactant mixture,the temperature of the mixture, and the amount of gaseous diluentselected from the group consisting of inert gases and 'a halogenadded'to the reactant mixture; such that for increased amounts ofhydrogen or increased velocity, or increased temperature of the mixture,a finer grain size is obtained, and'where at least one of the foregoingparameters is decreased, a larger grain size is obtained; and such thatwhen an inert gas is added to the mixture, a larger grain size isobtained, and when a halogen is added to the mixture, a finer grain sizeis obtained.

6. The process of claim 5, wherein a halogen gas is diluted with thereactant mixture to produce a finer grain size.

7. The process of claim 6, wherein the Halogen gasis chlorine. r

8. The process of claim 5, wherein an iner'tgas'selected from the groupconsisting of nitrogen and HCl is added 3,177,067 4/1965 Nichols 7s-0.sBB 3,341,320 9 1957 Smiley 75- 0.s BB 3,539,335, 11/1970, Svanstrom 7s0.s BB

U FOREIGN PATENTS 3 702,61; 1/1965 Canada 75 0-.5

L. DEWAYN'E RUTLEDGE, Primary Examiner W. W. STALLARD, AssistantExaminer- US: Cl. X.R. 75-05 BA, 01513 13, I

